A planetary gear train of an internal engagement type is known in the art. For example, the planetary gear train of the internal engagement type is used in a speed decreasing device, a speed increasing device and so on. The planetary gear train of the internal engagement type has an internal gear and an external gear, which is internally engaged with the internal gear.
In each of an internal gear and an external gear for a planetary gear train of an internal engagement type, which is disclosed in Japanese Patent No. 4,107,895, a runout portion is formed at a bottom portion of a gear tooth. A possible contact between the bottom portion of the internal gear and a top portion of the external gear as well as a possible contact between the bottom portion of the external gear and the top portion of the internal gear is avoided by the above runout portion when the gear tooth of the external gear and the gear tooth of the internal gear are located in a teeth engagement region, in which the gear teeth of the internal gear and the gear teeth of the external gear are in engagement between them.
According to the planetary gear train of the internal engagement type disclosed in the above prior art, however, it is not always possible to prevent a possible contact between the top portion of the gear tooth of the internal gear and the top portion of the gear tooth of the external gear in a teeth non-engagement region, in which the gear teeth of the internal gear and the gear teeth of the external gear are out of engagement from each other. As a result, it is a problem that torque transmission efficiency may be decreased because of the above contact.